Perovskite catalysts and uses thereof

ABSTRACT

The present disclosure provides perovskite catalytic materials and catalysts comprising platinum-group metals and perovskites. These catalysts may be used as oxygen storage materials with automotive applications, such as three-way catalysts. They are also useful for water or CO2 reduction, or thermochemical energy storage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of 62/421,568, filed Nov. 14, 2016,Zheng et al., entitled “Perovskite Catalysts and Uses Thereof” havingAtty. Dkt. 121-68-PROV which is hereby incorporated by reference in itsentirety.

1. FIELD

The present disclosure provides perovskite catalytic materials andcatalysts comprising platinum-group metals and perovskites. Thesecatalysts may be used as oxygen storage materials with automotiveapplications, such as three-way catalysts. They are also useful forwater or CO₂ reduction, or thermochemical energy storage.

2. BACKGROUND 2.1. Introduction

Perovskite materials have general chemical composition of ABO_(3-δ),where A and B are cations that may have multiple valence states, and arestable in octahedral and dodecahedral environments¹. The perovskitestructure is obtained when the Goldschmidt's tolerance factor (t) iswithin the range of 0.7<t<1, while t is calculated byt=(r_(A)+r_(O))√{square root over (2)}(r_(B)+r_(O)), where r_(A), r_(B),and r_(O) are the ionic radii for the A^(n+)B^(m+), and O^(2-, 2,3).Perovskite can exhibit significant swings in oxygen non-stoichiometry 6through the reaction with gas phase oxygen with the following reactionEq. 1⁴. δ may vary with temperature and gas phase oxygen partialpressure, and can be tailored by doping A and/or B sites withappropriately selected cations. The profound oxygen mobility withinperovskites makes them desired candidates for a wide variety ofapplications including water and/or CO₂ reduction, and thermochemicalenergy storage, and chemical-looping combustion⁴⁻⁹.

ABO_(3-δ)

ABO_(3-δ-Δδ)+½ΔδO₂  (1)

Oxygen storage material (OSM) is an essential component in modernthree-way catalysts (TWC) for automotive emissions control. Modern TWC,a supported bimetallic Pd—Rh catalyst deposited on stabilized γ-Al₂O₃,allows conversions of CO, hydrocarbons HCs, and NO_(x) to innocuouscompounds CO₂, H₂O, and N₂ ¹⁰. The TWC mode is operated within a narrowstoichiometric air-to-fuel ratio (λ=1), when the optimum conversions ofall three pollutants are reached ¹¹. The delicate control of the λ valueis chemically assisted by Ce_(x)O_(y), well known for its high oxygenstorage capacity (OSC) due to the function of Ce⁴⁺/Ce³⁺ redox pair. TheCe_(x)O_(y) is commercially incorporated into the γ-Al₂O₃ support toenhance the catalyst performance with the redox chemistry shown as Eq.(2) and (3).

2 CeO₂+CO→Ce₂O₃+CO₂ (at slightly fuel rich, λ<1)  (2)

Ce₂O₃+1/2 O₂→2 CeO₂ (at slightly fuel lean, λ>1)  (3)

In recent years the family of rare earths, including cerium, experienceda rapid increase in price and decrease in availability, upsetting themarkets and initiating a search for methods to reduce theirconcentrations in the OSC ¹². Meanwhile, with the rapid increase ofvehicle population and more stringent automotive emissions regulation,which requires as high as 96% conversion of all pollutants, improvementsof TWC performance by broadening operation window of the stoichiometricair-to-fuel ratios are of high interest ¹³. Furthermore, TWC, includingthe active metal (platinum group metals (PGM) metals) and the supportmaterials (with Ce_(x)O_(y)—ZrO₂-type OSM incorporated), may experiencedeactivation under fuel cutoff, an engine mode for enhanced fuel economybut exposes catalyst to a high temperature (c.a.>1000° C.) leanoxidative condition ^(13,14). Perovskites possess not only excellentredox property but also high chemical/structural stability ¹⁵.Preliminary development of perovskite-type OSMs by selecting transitionmetals with their low cost, large abundance and availability, remarkableredox properties, and good thermal stability would be a reasonableresearch strategy. Further research effort would require the developmentof advanced TWC by incorporation of PGM group metals into the novelperovskite structures with outstanding OSC capacity. Enhanced catalystactivity and stability were previously reported by incorporating noblemetals into the perovskite structures, which stabilize the metal againstsintering, reaction with the support, and volatilization ¹⁶⁻²⁰.

The oxygen mobility in perovskite increases with the amount of availableoxygen vacancies. Doping of A and B sites with cations influences theoxygen mobility by introducing structural defects (anionic or cationicvacancies) ²¹. Previous XRD analysis confirmed that doping Ca, Sr and Alcan be used to modify the La—Mn perovskite structure by varying thedopant type, position and doping extent ⁶. When the A-sites arepartially substituted by cations with lower valance, such as alkalimetal ions, the oxidation state of the B-site cations will be increasedor some oxygen vacancies are generated in order to maintain theelectrical neutrality ²². For example, when La³⁺ is partially replacedby Sr²⁺, the charge compensation is accomplished by increasing theunstable B ion amounts and oxygen vacancies, thereby facilitatingdiffusion of oxygen from bulk to surface ²³. The nature of the B-sitecation modifies the morphology, structural symmetry, the chargecompensation mechanism and the redox properties ³. For air-H₂ cyclicredox reactions, perovskites with La on A sites, and Co or Mn on B siteswere found to be the most active ^(9,24). Efforts were also made inorder to further increase activity by partial substitution of the Acation by cations Sr²⁺ and Ce⁴⁺ to modify the valency (Co³⁺/CO²⁺orMn⁴⁺/Mn²⁺) concentrations and oxygen mobility ¹. Klimkowicz, et al,reported that perovskite La_(0.5)Sr_(0.5)Co_(0.5)Fe_(0.5)O_(3-δ)exhibited enhanced reversible OSC than the state-of-the-artCe_(x)O_(y)—ZrO₂ (CZO) system ². Ran, et al, showed results that theNi-doped LaMnO_(3-δ) perovskite exhibited a very large dynamic OSC andhigh oxygen release rate, with Mn⁴⁺/Mn³⁺ redox pair contributing to thelow-temperature redox reaction ¹³. While these La-containingcompositions have high redox energy storage capacity, they are notcost-effective for applications at lower temperature (<1000° C.), e.g.TWC mode, where more cost-effective compositions with earth-abundantcations are preferred.

Furthermore, doping PGM metals (Pd, Rh, or Pt) at B-site will alsoenhance catalytic activity and bring a “self-regeneration” effect.²⁵ Itwas first reported by Tanaka and colleagues and supported by otherresearchers that in PGM-containing perovskite solid solutions, PGM metalwas reduced to a metallic state and disperse as small particles on thesurface after reduction, and returned to solid solution afterre-oxidation. This suggests that PGM-containing perovskite canregenerate itself under automotive fuel rich exhaust condition, andhence was named “An Intelligent Catalyst”.^(17,18,20,26) Excellentcatalytic activity and stability were since found with PGM-dopedperovskites, such as Pd doped La_(0.7)Sr_(0.3)CoO₃, LaAlO₃ and LaFeO₃,and Rh doped CaTiO₃, for HC, CO, and NO_(x) conversions at transientair/fuel ratio oscillation conditions and at various temperatures.²⁷⁻²⁹

U.S. Pat. No. 4,321,250 (Hart, Phillips Petroleum) discloses rhodium(Rh) containing perovskite-type catalysts with about 1 up to about 20percent of the B cation sites occupied by rhodium ions and the remainingB sites occupied by cobalt (Co). They disclose Catalyst A having theformula La_(0.8)Sr_(0.2)Co_(0.95)Rh_(0.05)O₃ and Catalyst A combinedwith an alumina support.

U.S. Pat. No. 6,680,036 (Fisher et al., Johnson Matthey) disclosesthree-way catalysts containing an oxygen storage component using a mixedoxide MnZr. They disclose catalysts loaded with palladium andcomparative data with CeZr catalysts.

US Published Appn. No. 2004/0024071 (Meier) discloses perovskites withGroup VIII metals (iron, cobalt, ruthenium, nickel) and their use ascatalysts for the conversion of synthesis gas to hydrocarbons.Specifically disclosed are LaFe_(0.5)Ti_(0.5)O₃, LaFe_(0.5)V_(0.5)O₃,LaFe_(0.5)Cr_(0.5)O₃, LaFe_(0.5)Mn_(0.5)O₃ and LaFe_(0.5)Zr_(0.5)O₃.

U.S. Pat. No. 7,718,562 (Gandhi et al., Ford Global Technologies)discloses two component catalysts with a first catalyst perovskite-basedand a second catalyst comprising precious metals. Specifically disclosedare La_(0.5)Ba_(0.5)Co_(0.9)Pt_(0.1)O₃,La_(0.5)Ba_(0.5)Co_(0.9)Rh_(0.1)O₃, La_(0.5)Ba_(0.5)Fe_(0.3)Pt_(0.1)O₃,

3. SUMMARY OF THE DISCLOSURE

The present disclosure provides in embodiment 1, a catalyst comprising aplatinum-group metal and a perovskite having the formulaCaCo_(1-x)Zr_(x)O_(3-δ) wherein x is a number defined by 0.02≤x≤0.98;and δ is a number defined by 0.0≤δ≤1.0.

In embodiment 2, the catalyst of embodiment 1, wherein theplatinum-group metal is Pd, Pt, Rh, Ru or a combination thereof.

In embodiment 3, the catalyst of embodiment 2, wherein theplatinum-group metal is a combination of Pd and Rh.

In embodiment 4, the catalyst of any of embodiment 1-3, wherein thecatalyst is on an Al₂O₃ support, a titania support, a zirconia support,a ceria support, a silica support, an alumina-silica support, a zeolitesupport, or a carbon support.

In embodiment 5, the catalyst of any of embodiment 1-4, wherein thecatalyst is formed into a monolith honeycomb block.

In embodiment 6, the catalyst of any of embodiment 1-4, wherein thecatalyst is coated on to a ceramic monolith honeycomb block.

In embodiment 7, the catalyst of embodiment 6, wherein the ceramicmonolith honeycomb block is a cordierite compound.

In embodiment 8, the catalyst of any of embodiment 1-7, wherein thecatalyst is a three-way catalyst.

In embodiment 9, the catalyst of any of embodiment 1-8, wherein thecatalyst is used to catalyze the reduction of NO_(N), and the oxidationof CO and hydrocarbons from an internal combustion engine.

In embodiment 10, the catalyst of embodiment 9, wherein the internalcombustion engine is an automobile engine.

In embodiment 11, the catalyst of embodiment 9, wherein the internalcombustion engine is operated under stoichiometric or lean air-to-fuelratio conditions.

In embodiment 12, the catalyst of embodiment 9, wherein the internalcombustion engine is fueled by diesel fuel, ethanol-gasoline hybridfuel, gasoline or natural gas.

In embodiment 13, the catalyst of embodiment 12, wherein theethanol-gasoline hybrid fuel is 85% ethanol 15% gasoline (E85).

In embodiment 14, a method for reducing emissions from an internalcombustion engine is provided which comprises contacting an exhauststream from the internal combustion engine with a catalyst comprising aplatinum-group metal and a perovskite having the formulaCaCo_(1-x)Zr_(x)O_(3-δ) wherein x is a number defined by 0.02≤x≤0.98;and δ is a number defined by 0.0≤δ≤1.0.

In embodiment 15, the method of embodiment 14, wherein theplatinum-group metal is Pd, Pt, Rh, Ru or a mixture thereof.

In embodiment 16, the method of any of embodiment 14-15, wherein thecatalyst is on an Al₂O₃ support.

In embodiment 17, the method of any of embodiment 14-16, wherein theinternal combustion engine is operated under stoichiometric air-to-fuelratio conditions.

In embodiment 18, the method of any of embodiment 14-17, wherein theinternal combustion engine is fueled by diesel fuel, ethanol-gasolinehybrid fuel, gasoline or natural gas.

In embodiment 19, an exhaust system for reducing emissions from aninternal combustion engine is provided which comprises a catalystcomprising a platinum-group metal and a perovskite having the formulaCaCo_(1-x)Zr_(x)O_(3-δ) herein x is a number defined by 0.02≤x≤0.98; andδ is a number defined by 0.0≤δ≤1.0 and a solid support.

In embodiment 20, a perovskite catalyst having the formulaCaCo_(1-x)Zr_(x)O_(3-δ) wherein x is a number defined by 0.02≤x≤0.98;and δ is a number defined by 0.0≤δ≤1.0.

In embodiment 21, the catalyst of embodiment 20 wherein x is a numberdefined by 0.2<x<0.8.

In embodiment 22, the catalyst of any of embodiment 19-21, wherein thecatalyst is in the form of a particle having a diameter greater thanabout 1.0 nm and less than about 10 mm.

In embodiment 23, the catalyst of embodiment 22, wherein the particlehas a diameter greater than about 1.0 μm and less than about 50 μm.

In embodiment 24, a method of preparing a perovskite catalyst having theformula CaCo_(1-x)Zr_(x)O_(3-δ) is provided wherein x is a numberdefined by 0.02≤x≤0.98; and 6 is a number defined by 0.0≤δ≤1.0, themethod comprising: (a) dissolving salts of Ca, Co and Zr to form ahomogenous solution; (b) drying the solution; and (c) calcining andsintering to form the perovskite catalyst.

In embodiment 25, the method of embodiment 24, wherein the calcining isat about 300° C. to about 500° C. and the sintering is at about 800° C.to about 1400° C.

In embodiment 26, a method of producing hydrogen by thermo-chemicalwater splitting is provided, the method comprising: (a) heating aperovskite catalyst having the formula CaCo_(1-x)Zr_(x)O_(3-δ) wherein xis a number defined by 0.02≤x≤0.98; and 6 is a number defined by0.0≤δ≤1.0 to release oxygen and generate an oxygen-depleted perovskitecatalyst; (b) contacting the oxygen-depleted particles with water torelease hydrogen and regenerate the perovskite catalyst.

In embodiment 27, the method of embodiment 26, wherein the hydrogen isproduced in a fluidized bed reactor.

In embodiment 28, the method of embodiment 27, wherein the fluidized bedreactor is a circulating fluidized bed reactor, a bubbling fluidized bedreactor, a transport reactor or a chemical looping reactor.

In embodiment 29, the method of embodiment 26, wherein the hydrogen isproduced in a fixed bed reactor.

In embodiment 30, the method of any of embodiment 26-29, wherein theperovskite catalyst is heated to a temperature of about 400° C. to about1000° C.

In embodiment 31, the method of any of embodiment 26-30, furthercomprising using the hydrogen produced in a subsequent reactor to reduceCO₂ to CO and H₂O.

In embodiment 32, the method of any of embodiment 26-30, furthercomprising using the hydrogen produced in a subsequent reactor to reduceCO₂ or CO to hydrocarbons and H₂O.

In embodiment 33, the method of any of embodiment 26-30, furthercomprising using the hydrogen produced in a subsequent reactor to reduceCO₂ or CO to alkanes or alkenes and H₂O.

In embodiment 34, the method of any of embodiment 26-30, furthercomprising using the hydrogen produced in a subsequent reactor toproduce aldehydes from mixtures of CO and alkenes.

In embodiment 35, the method of any of embodiment 26-30, furthercomprising using the hydrogen produced in a subsequent reactor forhydrotreating or hydroprocessing to upgrade crude or heavy petroleum orbiomass oil feedstocks.

In embodiment 36, the method of embodiment 31-33, wherein the CO₂ isproduced in a chemical looping combustion fuel reactor.

In embodiment 37, a method of reducing CO₂ to CO is provided, the methodcomprising: (a) heating a perovskite catalyst having the formulaCaCo_(1-x)Zr_(x)O_(3-δ) wherein x is a number defined by 0.02≤x≤0.98;and δ is a number defined by 0.0≤δ≤1.0 to release oxygen and generate anoxygen-depleted perovskite catalyst; (b) contacting the oxygen-depletedperovskite catalyst with CO₂ to remove oxygen, release CO and regeneratethe perovskite catalyst.

In embodiment 38, a system for the thermo-catalytic splitting of waterto produce hydrogen is provided, the system comprising: (a) an oxygenevolution reactor to heat a perovskite catalyst having the formulaCaCo_(1-x)Zr_(x)O_(3-δ) wherein x is a number defined by 0.02≤x≤0.98;and δ is a number defined by 0.0≤δ≤1.0 to generate an oxygen-depletedperovskite catalyst and release oxygen; (b) a hydrogen evolution reactorto react the oxygen-depleted perovskite catalyst with water vapor and toregenerate the perovskite catalyst and produce hydrogen; and (c) adevice configured to return the regenerated perovskite catalyst to theoxygen evolution reactor.

In embodiment 39, the system of embodiment 38, wherein the oxygenevolution reactor is a fluidized bed reactor.

In embodiment 40, the system of any of embodiment 38-39, wherein thehydrogen evolution reactor is a fluidized bed reactor.

In embodiment 41, the system of embodiment 38, wherein the hydrogenevolution reaction fluidized bed is a riser reactor.

In embodiment 42, the method of embodiment 38, wherein either the oxygenevolution reactor or the hydrogen evolution reactor is a fixed bedreactor.

In embodiment 43, the system of any of embodiment 38-42, wherein thehydrogen is used to reduce CO₂ in an exhaust gas from a combustionprocess.

In embodiment 44, the system of any of embodiment 38-43, wherein thehydrogen is used for thermal energy storage.

In embodiment 45, the system of any of embodiment 38-44, wherein thesystem is integrated into a chemical manufacturing system and facilitythat provides energy for water splitting reactions while minimizing theenergy losses.

In embodiment 46, a catalyst is provided comprising a platinum-groupmetal and a perovskite having the formula(La_(1-y)Ca_(y))_(1-x)Mn_(x)O_(3-δ),La_(1-x)(Co_(1-y)Ru_(y))_(x)O_(3-δ),(La_(1-y)Sr_(y))_(1-x)Co_(x)O_(3-δ),Sr_(1-x)(Co_(1-y)Fe_(y))_(x)O_(3-δ),(Sr_(1-y)Ca_(y))_(1-x)Fe_(x)O_(3-δ); x is a number defined by0.02≤x≤0.98; y is a number defined by 0.02<y<0.98; and 6 is a numberdefined by 0.0<6<1.0.

In embodiment 47, the catalyst of embodiment 46, wherein y is a numberdefined by 0.15≤y≤0.85.

In embodiment 48, the catalyst of any of embodiment 46-47, wherein x isa number defined by 0.3≤x≤0.7.

In embodiment 49, the catalyst of any of embodiment 46-48, wherein theplatinum-group metal is Pd, Pt, Rh, Ru, or a mixture thereof.

In embodiment 50, the catalyst of embodiment 49, wherein theplatinum-group metal is a mixture of Pd and Rh.

In embodiment 51, the catalyst of any of embodiment 46-50, wherein thecatalyst is on an Al₂O₃ support, a titania support, a zirconia support,a ceria support, a silica support, an alumina-silica support, a zeolitesupport, or a carbon support.

In embodiment 52, the catalyst of any of embodiment 46-51, wherein thecatalyst is formed into a monolith honeycomb block.

In embodiment 53, the catalyst of any of embodiment 46-52, wherein thecatalyst is coated on to a ceramic monolith honeycomb block.

In embodiment 54, the catalyst of embodiment 53, wherein the ceramicmonolith honeycomb block is a cordierite compound.

In embodiment 55, the catalyst of any of embodiment 46-54, wherein thecatalyst is a three-way catalyst.

In embodiment 56, the catalyst of any of embodiment 46-54, wherein thecatalyst is used to catalyze the reduction of NO_(x) or the oxidation ofCO or hydrocarbons from an internal combustion engine.

In embodiment 57, the catalyst of embodiment 56, wherein the internalcombustion engine is an automobile engine.

In embodiment 58, a method for reducing emissions from an internalcombustion engine is provided which comprises contacting an exhauststream from the internal combustion engine with a catalyst comprising aplatinum-group metal and a perovskite having the formula(La_(1-y)Ca_(y))_(1-x)Mn_(x)O_(3-δ),La_(1-x)(Co_(1-y)Ru_(y))_(x)O_(3-δ),(La_(1-y)Sr_(y))_(1-x)Co_(x)O_(3-δ),Sr_(1-x)(Co_(1-y)Fe_(y))_(x)O_(3-δ),(Sr_(1-y)Ca_(y))_(1-x)Fe_(x)O_(3-δ); x is a number defined by0.02≤x≤0.98; y is a number defined by 0.02≤y≤0.98; and 6 is a numberdefined by 0.0≤δ≤1.0.

In embodiment 3, an exhaust system for reducing emissions from aninternal combustion engine is provided which comprises a catalystcomprising a platinum-group metal and a perovskite having the formula(La_(1-y)Ca_(y))_(1-x)Mn_(x)O_(3-δ),La_(1-x)(Co_(1-y)Ru_(y))_(x)O_(3-δ),(La_(1-y)Sr_(y))_(1-x)Co_(x)O_(3-δ),Sr_(1-x)(Co_(1-y)Fe_(y))_(x)O_(3-δ),(Sr_(1-y)Ca_(y))_(1-x)Fe_(x)O_(3-δ); x is a number defined by0.02≤x≤0.98; y is a number defined by 0.02≤y≤0.98; and 6 is a numberdefined by 0.0≤δ≤1.0 and a solid support.

4. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A-1B. X-ray diffraction (XRD) patterns of (FIG. 1A)state-of-the-art perovskite samples with different A- and B-site ions,(FIG. 1B) RTI's novel CaCo_(x)Zr_(1-x)O_(3-δ) perovskite samples withand without Pd doping.

FIG. 2. Oxygen non-stoichiometry (δ) as a function of sample temperatureof as-synthesized perovskite samples, in comparison with commercial CZOand CeO₂. The result is calculated by using temperature programmedthermogravimetric (TG) data, assuming the onset temperature of thermaloxygen release is 500° C. (following complete sample degas).

FIG. 3A-3B. CO₂ production and CO intake vs. reaction time on stream(TOS) and temperature of studied samples as represented by MS signals(in partial pressure) recorded during CO-TPR/MS measurements.

FIG. 4. H₂ consumption as a function of reaction time on stream andtemperature of studied samples during H₂-TPR measurements (temperatureramp rate at 5° C./min).

FIG. 5A-5B. CO₂ production and 02 intake vs. reaction time on stream(TOS) of PE-1 to PE-6 perovskites, in comparison to CeO₂ and CZO, asrepresented by MS partial pressure signals during isothermal COreduction-Air oxidation cyclic tests at 500, 600, 700, and 800° C.

FIG. 6. CO₂ production and 02 intake vs. reaction time on stream (TOS)of PE-6 to 9 perovskite samples (CaCo_(x)Zr_(1-x)O_(3-δ), with x=0.3,0.5, 0.7, or 0.9), as represented by MS partial pressure signals duringisothermal CO reduction-Air oxidation cyclic tests at 500, 600, 700, and800° C.

FIG. 7A-7C. Plots for oxygen-intake kinetic calculation ofCaCo_(0.5)Zr_(0.5)O_(3-δ) perovskite sample showing (FIG. 7A) Fractionreacted a with time variation, (FIG. 7B) Fraction reacted a againstreduced time t/t_(0.5), and (FIG. 7C) ln[−ln(1−a)] as a function of ln t(with t in min) for the determination of reaction mechanisms attemperatures varied from 350° C. to 475° C.

FIG. 8. Temperature dependent-reaction rate constants k in natural logscale as a function of inverse temperature for the calculation ofreaction activation energy for CaCo_(0.5)Zr_(0.5)O_(3-δ) perovskite. Thek value is achieved by fitting reaction data with various kineticmodels, and first-order kinetic model was selected.

FIG. 9. (1) CO and (2) C₃H₈ conversions during catalytic oxidation testsover (Panel 1a/2a) CaCo_(0.5)Zr_(0.4)Pd_(0.1)O_(3-δ), (Panel 1b/2b)CaCo_(0.55)Zr_(0.4)Pd_(0.05)O_(3-δ), and (Panel 1c/2c)CaCo_(0.5)Zr_(0.5)O_(3-δ) perovskite with simulated exhaust feed atstoichiometric numbers (SNs) of 1.16, 1.07 and 0.95.

5. DETAILED DESCRIPTION OF THE DISCLOSURE 5.1. Definitions

While the following terms are believed to be well understood by one ofordinary skill in the art, the following definitions are set forth tofacilitate explanation of the presently disclosed subject matter.

The term “oxygen storage material” (OSM) as used herein means a stablematerial that has the capacity to both reversibly take up with gas phaseoxygen and release the oxygen. In one embodiment, such as automobileexhaust, OSMs may take up oxygen from NO_(x) or release oxygen for theoxidation of hydrocarbons or carbon monoxide. The perovskite catalystsdescribed herein are OSMs. The perovskite catalysts may be combined withconventional OSMs such as Ce_(x)O_(y)—ZrO₂ (CZO) mixed oxides.

The term “perovskite” means a metal oxide of the formula ABO_(3-a) orA¹A²B¹B²O_(3-δ) having a cubic crystalline form. The cations A and B aremetals that may have multiple valence states, and are stable in bothoctahedral and dodecahedral environments.

The term “platinum-group metal” or (PGM) means a group VIII metal fromthe periodic table. Preferred PGMs are Pd, Pt, Rh, Ru or combinationsthereof.

The term “three-way catalyst” or (TWC) means a substance that enablesthe oxidation of CO, unburnt hydrocarbons (HCs) or the reduction ofNO_(x) to N₂ to proceed at a usually faster rate or at a lowertemperature. The three chemical reactions may be simultaneous or mayoccur in a staged catalytic system such as the TWC systems disclosed inU.S. Pat. No. 7,718,562 where a first catalyst reduces the NO_(x) and asecond catalyst oxidizes the CO and HCs. In a staged catalytic system,the perovskites disclosed herein may be present as a component of eitherthe first stage catalyst, the second stage catalyst, or both stagecatalysts.

Throughout the present specification, the terms “about” and/or“approximately” may be used in conjunction with numerical values and/orranges. The term “about” is understood to mean those values near to arecited value. For example, “about 40 [units]” may mean within ±25% of40 (e.g., from 30 to 50), within ±20%, ±15%, ±10%, ±9%, ±8%, ±7%, ±6%,±5%, ±4%, ±3%, ±2%, ±1%, less than ±1%, or any other value or range ofvalues therein or there below. Furthermore, the phrases “less than about[a value]” or “greater than about [a value]” should be understood inview of the definition of the term “about” provided herein. The terms“about” and “approximately” may be used interchangeably.

Throughout the present specification, numerical ranges are provided forcertain quantities. It is to be understood that these ranges compriseall subranges therein. Thus, the range “from 50 to 80” includes allpossible ranges therein (e.g., 51-79, 52-78, 53-77, 54-76, 55-75, 60-70,etc.). Furthermore, all values within a given range may be an endpointfor the range encompassed thereby (e.g., the range 50-80 includes theranges with endpoints such as 55-80, 50-75, etc.).

As used herein, the verb “comprise” as is used in this description andin the claims and its conjugations are used in its non-limiting sense tomean that items following the word are included, but items notspecifically mentioned are not excluded.

Throughout the specification the word “comprising,” or variations suchas “comprises” or “comprising” will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps. The present disclosure maysuitably “comprise”, “consist of” or “consist essentially of”, thesteps, elements, and/or reagents described in the claims.

It is further noted that the claims may be drafted to exclude anyoptional element. As such, this statement is intended to serve asantecedent basis for use of such exclusive terminology as “solely”,“only” and the like in connection with the recitation of claim elements,or the use of a “negative” limitation.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art to which this disclosure belongs. Preferred methods, devices,and materials are described, although any methods and materials similaror equivalent to those described herein can be used in the practice ortesting of the present disclosure. All references cited herein areincorporated by reference in their entirety.

The following Examples further illustrate the disclosure and are notintended to limit the scope. In particular, it is to be understood thatthis disclosure is not limited to the particular embodiments described,which as such may, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting, since the scope ofthe present disclosure will be limited only by the appended claims.

6. EXAMPLES

The present study focuses on a systematic screening of advancedperovskites as potential substitutes for commercial CZO in TWC structurefor automotive applications. Some perovskite compositions used formaterial synthesis were selected based on the most recent literaturereporting profound material performances in other applications ³⁰⁻³⁴.CaCo_(x)Zr_(1-x)O_(3-δ) perovskites were synthesized for the first timeand reported here to show outstanding redox property and oxygen mobilitycompared to the current state-of the-art perovskite materials and thecommercial CZO. CO— and H₂-temperature programmed oxidationmeasurements, and dynamic CO-air redox cyclic tests were performed toexamine the CO and oxygen storage capacities of the as-synthesizedmaterials, in comparison with the commercial CZO and CeO₂. Oxygennon-stoichiometry measurements were applied to study the correlationbetween temperature-dependent oxygen vacancy population and the materialredox property. X-ray fluorescence, X-ray diffraction, and BET surfacearea measurements were used to characterize the studied materials. Thekinetic data of the oxygen intake of the best performing perovskitematerials is included. The study aims at providing a comprehensivecomparison of the oxygen storage/release capacities of the currentstate-of-the-art perovskite materials, and preliminary results for thedevelopment of high performing perovskite materials in automotive andother potential applications.

2. EXPERIMENTAL

2.1. Perovskite Material Synthesis

Perovskites with target compositions of CaCo_(x)Zr_(1-x)O_(3-δ)(x=0,0.3, 0.5, 0.7 and 0.9) and Pd-dopedCaCo_(0.6-y)Zr_(0.4)Pd_(y)O_(3-δ)(y=0.05, 0.1 and 0.2) were synthesizedusing Pechini method.³⁵ Calcium nitrate tetrahydrate (Sigma-Aldrich, St.Louis, Mo.), cobalt (II) nitrate hexahydrate (STREM Chemical,Newburyport, Mass.), zirconium (IV) oxynitrate hydrate (Sigma-Aldrich)and palladium (II) nitrate dihydrate (Alfa Aesar, Tewksbury, Mass.) wereused as the metal precursors. Stoichiometric amounts of metal nitratesalts were first dissolved in ethylene glycol (Acros Organics,Pittsburgh, Pa.), followed by the addition of citric acid (FisherChemical, Pittsburgh, Pa.), with input molar ratios of n(metal):n(citricacid):n(ethylene glycol)=2:3:9. A solution was formed at 150° C. andthen was slowly heated to 250° C. under continuous stirring till ahomogeneous gel was formed, which was then cooled to RT. Calcination wasthen performed at 400° C. for 4 hrs, followed by sintering at 1200° C.for 4 hrs in a high temperature oven with constant dry air flow (5mL/min). Commercial Ce_(x)O_(y)—ZrO₂ (CZO, with Ce:Zr molar ratio of1:4) and CeO₂ were respectively acquired from MEL Chemical (Flemington,N.J.) and Alfa Aesar. Other perovskites includingLa_(0.6)Sr_(0.4)CoO_(3-δ), Sr_(0.76)Ca_(0.24)FeO_(3-δ),LaCo_(0.6)Ru_(0.4)O_(3-δ), SrCo_(0.85)Fe_(0.15)O_(3-δ), andLa_(0.5)Ca_(0.5)MnO_(3-δ) were synthesized according to literature.³⁰⁻³⁴

2.2. Oxygen Non-Stoichiometry Study

The temperature-dependent oxygen non-stoichiometry (6) of theas-synthesized perovskites samples were studied by temperatureprogrammed thermogravimetric analysis (TGA) using a TA Q500 unit. Around20 mg sample was first loaded onto a platinum pan, and degassed at 500°C. for 2 hrs in He at a flow rate of 60 mL/min. The weight loss of thedegassed sample was then measured with increasing temperature from 500°C. to 900° C. at ramp rate of 5° C./min in helium at the same flow rate.The oxygen non-stoichiometry was correlated to the sample weight loss,assuming 500° C. was the onset temperature of the thermal oxygenrelease. The imaginary oxygen non-stoichiometry of commercialCe_(x)O_(y)—ZrO₂ (CZO, with Ce:Zr molar ratio of 1:4, MEL Chemical) andCeO₂ (Alfa Aesar) were also calculated in comparison with the studiedperovskites. A background measurement with an empty platinum pan wasperformed with the same reaction program to take into account thebuoyancy effect.

2.3. CO-Temperature Programmed Reduction Measurements

CO-temperature programmed reduction (CO-TPR) measurements of theas-synthesized perovskite and commercial samples were performed using aMicromeritics AutoChem II 2920 reactor equipped with a built-in TCDdetector, and with the reactor downstream connected to a benchtopquadrupole mass spectrometer (TA Discovery Mass Spectrometer). The TCDsignal was used for the quantification of the CO intake, while the MSsignal was used for product gas identification and semi-quantifiableanalysis. Around 0.5 g sample sandwiched in thin quartz wool was firstloaded into a U-shaped quartz reactor, which was housed in a thermalfurnace with upper temperature limit of 1000° C. After degassing at 500°C. for 1 hr in He at a flow rate of 50 mL/min, the sample was heatedfrom ambient temperature (25° C.) to 900° C. at 5° C./min in 20% CO/He,at a flow rate of 30 mL/min, while the TCD and MS responses wererecorded every 0.1 second. The TCD calibration for the total signal(reduced CO and increased CO₂ amount) was performed by using both the COand CO₂ linear calibration curves, and 20% CO/He was used as thereference gas. A background test with an empty reactor and the samereaction conditions was performed to create a baseline.

2.4. H₂-Temperature programmed reduction measurements

H₂-temperature programmed reduction (H₂-TPR) measurements of theas-synthesized perovskite and commercial samples were performed usingthe same above mentioned AutoChem reactor system. Around 0.5 g samplewas first degassed at 500° C. for 1 hr in He at a flow rate of 50mL/min, and then heated from ambient temperature (25° C.) to 900° C. at5° C./min in 10% H₂/Ar, at a flow rate of 30 mL/min. The water generatedduring TPR was trapped instantly downstream to the reactor by using apropanol-liquid N₂ mixture coolant. The TCD calibration was performedwith H₂ concentrations varied from 0 to 10% in Ar, and with 10% H₂/Ar asthe reference gas. A background test with an empty reactor and the samereaction conditions was performed to create a baseline.

2.5. Dynamic Measurements of CO and Oxygen Storage Capacities

The oxygen mobility within the studied samples were further examinedusing dynamic isothermal CO-air redox cyclic tests by using the sameabove mentioned AutoChem reactor system. Samples were first degassed inHe (50 mL/min) at 500° C. for 2 hrs before cyclic tests. Each cycle wasperformed following the CO reduction-air oxidation order, respectivelyat 500° C., 600° C., 700° C., and 800° C., with 3 repetitive cycles ateach reaction temperature. During the reduction, 20% CO/He flowedthrough the sample at a constant flow rate of 30 mL/min. During theoxidation, air (moisture removed) flowed through the sample at aconstant flow rate of 30 mL/min. The reduction and oxidation durationswere both kept at 0.5 min (30 sec). In between reduction and oxidationsteps, He at a flow rate of 50 mL/min was used to purge the systemresidual gas. The CO₂ production during the reduction cycles, and the O₂uptakes during the oxidation cycles were closely monitored by TCD and MSsignals. A background test with an empty reactor was performed at thesame cyclic conditions.

2.6. Isothermal Thermogravimetric Experiments for Reaction Kinetic Study

The reaction kinetics of best performing perovskite was studied byisothermal TG experiments using a TA Q500 unit. Approximately 20 mg ofeach studied sample was degassed at 500° C. in He (at 50 mL/min) for 1hr, followed by pre-reduction in 20% CO/He (at 30 mL/min) for 1 hr atthe same temperature. The pretreated sample was then placed in a shallowplatinum pan and was heated to a target temperature (350, 375, 400, 425,450, 475, or 500° C.) in flowing He at 60 mL/min. The flow was thenswitched to 5% 02/He to gradually oxidize the sample after the targettemperature was reached and a stable baseline was achieved, while thesample weight signal was recorded every 0.1 second during the wholeprocess. A background measurement with empty platinum pan was performedat each reaction temperature to take into account the buoyancy effect.The kinetic models were adopted from Hancock's [29] and Motohashi's [30]previous studies, with major calculation equations describedaccompanying the corresponding body text.

2.7. Catalyst Characterization

2.7.1. X-Ray Fluorescence (XRF) Analysis

X-ray fluorescence analyses (XRF) for elemental compositional study ofthe studied materials were performed with ARL Thermo Scientific PERFORMXWavelength-Dispersive X-ray Fluorescence (WDXRF) instrument. A 5GN-typeRh target incident beam with ultra-thin 30 μm Be window was used tomaximize light element response. The instrument was equipped with twodetectors and seven analyzer crystals to achieve a broad elementalrange. Sample data was processed using UniQuant, a standard softwarepackage that uses advanced fundamental parameters algorithms todetermine elemental concentrations. Analysis is for seventy-nineelements and those elements above ten times the instrument calculateduncertainty are reported.

2.7.2. X-Ray Diffraction (XRD) Analysis

The phase identification was done by means of X-ray diffraction (XRD)analyses of the studied materials with a Thermo INEL Equinox 100benchtop X-ray diffractometer. XRD patterns were collected withmonochromatized Cu Kα radiation over the 20 range of 20−70° with a totalscanning duration of 1200 seconds at an instrument power setting of 40kV and 0.9 mA. The scans were performed in validation of the instrumentusing the vendors Y₂O₃ standard yielded peak accuracy as compared toICDD PDF reference values that fell well within the SOP-statedacceptable values of 0.05° of 2θ. All measurements were processed usingJade software.

2.7.3. BET Surface Area Analysis

BET surface areas of the studied materials were measured using an ASAP2020 Plus Chemisorption unit. Each sample (around 0.2 g) was firstdegassed in vacuum (<500 μm Hg) at 350° C. for 4 hrs. The Multi-pointBET surface area of the degassed sample was measured under partialpressure P/P₀ of 0.05 to 0.30 at liquid nitrogen temperature (77 K).

3. RESULTS AND DISCUSSION

3.1. Structural Analysis of Studied Materials

The actual compositions of the as-synthesized perovskite samples wereanalyzed by XRF with replicate results within an error limit of <10%.The target (theoretical) and actual compositions of the studiedmaterials are listed in Table 1. The differences between the two valuesfor all perovskites are within allowable ranges. The analyticalinformation of the commercial Ce_(x)O_(y)—ZrO₂ (CZO) and CeO₂ areprovided by their manufactures. Table 1 also lists the tolerance factors(t) of the perovskites, calculated based on the theoretical materialcompositions. The t values were calculated based on the ionic radii ofCa²⁺, La³⁺, Sr²⁺ for A-site ions, and Co³⁺, Fe³⁺, Mn³⁺, Ru³⁺, Zr⁴⁺, andPd²⁺ for B-site ions. The t values of PE-1 to 9 samples are within therange of 0.8˜0.92, suggesting stable orthorhombic perovskitestructures.³

TABLE 1 Analytical data of as-synthesized ABO_(3-δ) perovskite samples,and commercial Ce_(x)O_(y)—ZrO₂ (CZO) and CeO₂ samples. # Targetcomposition Actual composition^(a) t^(b) Ā (m²/g)^(c) PE-1La_(0.6)Sr_(0.4)CoO_(3-δ) La_(0.56)Sr_(0.47)Co_(1.03)O_(3-δ) 0.91 1.16PE-2 Sr_(0.76)Ca_(0.24)FeO_(3-δ) Sr_(0.84)Ca_(0.21)Fe_(0.96)O_(3-δ) 0.922.36 PE-3 LaCo_(0.6)Ru_(0.4)O_(3-δ) La_(0.99)Co_(0.49)Ru_(0.44)O_(3-δ)0.86 2.58 PE-4 SrCo_(0.85)Fe_(0.15)O_(3-δ)Sr_(1.10)Co_(0.82)Fe_(0.14)O_(3-δ) 0.94 2.59 PE-5La_(0.5)Ca_(0.5)MnO_(3-δ) La_(0.50)Ca_(0.52)Mn_(1.08)O_(3-δ) 0.89 2.09PE-6 CaCo_(0.3)Zr_(0.7)O_(3-δ) Ca_(1.33)Co_(0.37)Zr_(0.52)O_(3-δ) 0.821.88 PE-7 CaCo_(0.5)Zr_(0.5)O_(3-δ) Ca_(1.17)Co_(0.61)Zr_(0.36)O_(3-δ)0.84 3.19 PE-8 CaCo_(0.7)Zr_(0.3)O_(3-δ)Ca_(1.12)Co_(0.79)Zr_(0.20)O_(3-δ) 0.85 2.21 PE-9CaCo_(0.9)Zr_(0.1)O_(3-δ) Ca_(1.08)Co_(0.98)Zr_(0.07)O_(3-δ) 0.86 1.76PE-10 CaCo_(0.4)Zr_(0.4)Pd_(0.2)O₃Ca_(1.12)Co_(0.0.43)Zr_(0.33)Pd_(0.25)O₃ 0.82 0.05 PE-11CaCo_(0.5)Zr_(0.4)Pd_(0.1)O₃ Ca_(1.12)Co_(0.55)Zr_(0.32)Pd_(0.11)O₃ 0.830.03 PE-12 CaCo_(0.55)Zr_(0.4)Pd_(0.05)O₃Ca_(1.16)Co_(0.61)Zr_(0.32)Pd_(0.06)O₃ 0.84 0.15 CZO^(d) Ce:Zr molarratio 1:4 Ce:Zr molar ratio 1:4 N/A 215.80 CeO₂ ^(e) CeO₂ CeO₂ N/A 15.52Annotations: ^(a)Material actual composition as measured by XRF, withrepeatable results and acceptance limits of less than 10%;^(b)Theoretical tolerance factor (in the range of 0.75 to 1) for theperovskite material sample calculated based on the target compositions;^(c)Multi-point specific BET surface areas Ā measured at liquid N₂temperature (77 K), at relative pressures (P/P₀) in the range of 0.05 to0.30; ^(d)Commercial CZO sample acquired from MEL Chemical, withmaterial composition information provided; ^(e)Commercial CeO₂ sampleacquired from Alfa Aesar, with material composition informationprovided.

More detailed material structural information of the studied materialscan be obtained from XRD analyses. FIG. 1A-1B plots the highly resolvedXRD patterns of all the fresh samples. FIG. 1A presents the XRD patternsof as-synthesized perovskite samples PE-1 to 5 with different A- andB-site ions. The XRD pattern for La_(0.6)Sr_(0.4)CoO_(3-δ) (PE-1) isconsistent with database (PDF#01-070-7597) and previous report ³⁰, andis characteristic of the rhombohedral lattice structure. All thediffraction peaks for Sr_(0.76)Ca_(0.24)FeO_(3-δ) (PE-2) can be assignedto perovskite-type structure (PDF#01-082-2445) ³¹. The diffractionpattern of the LaCo_(0.6)Ru_(0.4)O_(3-δ) sample (PE-3) show profilescorresponding to single perovskite structures (PDF#01-082-9769) withoutpeaks attributable to ruthenium oxides ³². The XRD pattern ofSrCo_(0.85)Fe_(0.15)O_(3-δ) sample (PE-4) also agrees well with thepreviously reported perovskite structure without any impurity phases(PDF#04-014-2297)³³. La_(0.5)Ca_(0.5)MnO_(3-δ), (PE-5) showed distinctpeaks which correspond well to previously reportedLa_(0.67)Ca_(0.33)MnO₃ perovskite (PDF#04-014-6391).

The XRD patterns of the PE 6-12 novel perovskite samples with or withoutPd doping are shown in FIG. 1B. Reflections at 20 of 22.1, 31.0, 31.5,32.0, 45.2, 50.2, 50.9, 51.6, 55.2, 55.9, 56.5, 56.7, 64.7, 65.8 and66.9 were assigned to the main phase of orthorhombic Lakargiite CaZrO₃perovskite matrix (JCPDS 01-080-6213); reflections at 20 of 30.3, 35.1,50.5, 60.0 and 63.0 were assigned to cubic Tazheranite ZrO₂ structure(JCPDS 04-002-8314); reflections at 20 of 19.5, 32.5, 34.2, 41.6, 42.3,46.4 and 47.8 were assigned to Ca₃Co₂O₆ phase (JCPDS 04-010-0812); andreflections at 20 of 40.2 and 46.7 were assigned to Pd phase (JCPDS01-089-4897).

For CaCo_(x)Zr_(1-x)O₃ samples, with lower x values (higher Zr/Co ratioat B sites), higher perovskite main phase crystallinity was observed,while increased amount of ZrO₂ phase was detected. When x>0.7, notableamount of Ca₃Co₂O₆ phase was detected. This suggests that partialsubstitution of Co by Zr enhances the structural crystallinity ofCaCoO₃. With x value of around 0.5, CaCo_(0.5)Zr_(0.5)O₃ processesoptimum crystallinity and minimized impurity. Noteworthy, no side phasesof Cobalt Oxides in any single form were identified when x<0.7,suggesting the Co was inside the perovskite unit cells.

For CaCo_(0.6-y)Zr_(0.4)Pd_(y)O₃ samples, the main phase remainedCaCo_(x)Zr_(1-x)O₃ perovskite, as the characteristic peaks matched thoseobserved with CaCo_(0.5)Zr0.5O3. Peaks characterizing Pd were observed,and the Pd phase amount increased with increasing Pd loading. No peakcorresponding to PdO was shown. This suggests that among the doped Pd,some were incorporated into the perovskite bulk crystal structure (bulkPd²⁺), while others remained on the crystal surfaces (surface Pd⁰). Theexistence of bulk Pd²⁺ will be further evidenced by H₂-TPR result in thefollowing text.

3.2. Temperature-Dependent Oxygen Non-Stoichiometry of as-SynthesizedPerovskites

The temperature-dependent oxygen non-stoichiometry (5) of theas-synthesized perovskites and the commercial CeO₂ and CZO samples arecompared in the temperature range of 500° C. to 900° C. in FIG. 2.Generally, 6 values increase with increasing temperature. Remarkabledifferences in the δ increase rate are observed between perovskite andCe-containing non-perovskite samples. Compared to perovskite samples,CeO₂ and CZO showed negligible variations in δ values with temperature.Among perovskite samples, PE-1 to 5 showed steady 6 increase rate duringthe entire temperature programmed process, where PE-1(La_(0.6)Sr_(0.4)CoO_(3-δ)) was the leading material. Compared to PE-1to 5, the oxygen release of CaCo_(x)Zr_(1-x)O_(3-δ)— type perovskiteswas a more thermally activated process, with δ values observed toincrease exponentially at 780° C. Specifically, pronounced amount ofoxygen vacancies were created in CaCo_(x)Zr_(1-x)O_(3-δ) when x>0.3 whentemperature reached 800˜900° C.

3.3. Reducibility of as-Synthesized Perovskites in Comparison toCommercial Ceria-Based OSM

CO-TPR/MS profiles of the as-synthesized perovskites and commercialceria-based oxygen storage material (OSM) samples are presented by FIG.3. No oxidation pretreatment was performed considering that the freshsamples as they were already exposed to oxidative calcinationenvironments during material synthesis, i.e. 1200° C. in air forperovskite samples and around 550° C. in air for commercial CeO₂ and CZOsamples. Complete sample degas pretreatments were applied prior to allthe measurements. FIG. 3A-3B plots the semi-quantitative MS signals, interms of partial pressure (in Torr) of the mass fraction, ofrespectively (FIG. 3A) CO₂ (mass of 44) and (FIG. 3B) CO (mass of 28) inthe CO-TPR product stream. The CO₂ and CO baselines achieved by flowingthe same reactant gas (10% CO/He) through an empty reactor tube at thesame temperature conditions are also plotted. The positive CO₂ peaks andnegative CO peaks compared to baseline indicate sample reduction by COas reduction temperature ramps from ambient to as high as 900 ^(o)C.Higher extent of sample reduction is represented by higher intensity ofCO₂ signal and higher absolute value of the CO intensity.

In FIG. 3A-3B, highly resolved MS signals of CO₂ and CO recorded duringCO-TPR measurements suggest the efficacy of the analysis approach. Theonset reduction temperatures of the studied samples were around 150° C.to 250° C., and the reduction extends as temperature increases to 900°C. For each sample, the outstanding TPR peaks at different temperatureranges represent varied types of solid reductions. For the perovskites,the reduction mainly occurred with the B cations (Co^(m+), Fe^(n+,)Mn^(P+,) and Ru^(q+)) since all the A cations (Ca²⁺, La³⁺ and Sr²⁺) arenon-reducible ³ at the described conditions. More specifically, thereduction happened with the surface oxygen-stabilized B site ions. Anexception was with CaCo_(x)Zr_(1-x)O_(3-δ) perovskite samples (PE-6 to9), in which B site Zr⁴⁺ ions were non-reducible at the describedconditions, while the incorporation of Zr⁴⁺ was simply for structuraltuning reason as previously discussed.

Generally, the volumetric amounts of CO consumptions (V _(CO), mL/gsample) follow the trend of CaCo_(x)Zr_(1-x)O_(3-δ)(x=0.9, 0.7, and0.5)>LaCo_(0.6)Ru_(0.4)O_(3-δ)>CaCo_(0.3)Zr_(0.7)O_(3-δ)>La_(0.6)Sr_(0.4)CoO_(3-δ)>La_(0.5)Ca_(0.5)MnO_(3-δ)>CeO₂>CZO>SrCo_(0.85)Fe_(0.15)O_(3-δ)>Sr_(0.76)Ca_(0.24)FeO_(3-δ).The reducibility of all the as-synthesized perovskite samples are higherthan the commercial CeO₂ and CZO samples. Among the studied perovskites,CaCo_(x)Zr_(1-x)O_(3-δ) exhibits good reducibility in CO-TPR.Specifically, CaCo_(0.9)Zr_(0.1)O_(3-δ) shows significantly enhancedreducibility than the current state-of-the-artLa_(0.6)Sr_(0.4)CoO_(3-δ).

FIG. 4 presents the H₂— TPR profiles of CaCo_(x)Zr_(1-x)O₃ andCaCo_(0.6-y)Zr_(0.4)Pd_(y)O₃. The TPR profiles of CaCo_(x)Zr_(1-x)O₃(except for CaZrO₃) show two successive reduction peaks, with one in therange of 280-500° C., and the other in the range of 450-680 ^(o)C.Consistent with previous studies, the two peaks can be respectivelyassigned to the reduction of Co³⁺to Co²⁺, and Co²⁺to Co^(0,36,37) Theamount of reducible sites increased with increasing Co contents. CaZrO₃showed no reduction at all, which further proved that the reductionsoccurred solely on the Co sites. On the other hand, the reductiontemperatures for both peaks shift to lower values with increasing Zrcontent. This result indicates that B-site substitution with Zr enhancedthe reduction of Co species by lowering the reduction temperature.

The H₂-TPR profiles of fresh Pd-doped samplesCaCo_(0.6-y)Zr_(0.4)Pd_(y)O_(3-δ) show obvious shifts of thehigh-temperature Co reduction peak (Co³⁺to Co²⁺) to lower temperature(80-100° C. lower). Noticeable shifts to lower temperature of theCo²⁺-to-Co⁰ reduction peaks were also observed with Pd-doping. Moreover,the shifts were more significant with increasing Pd content. With thehighest amount of Pd loading, three overlapping peaks are seen withCaCo_(0.4)Zr_(0.4)Pd_(0.2)O₃. The successive two reduction peaks athigher temperatures (T_(max)=390° C. and 420° C.) can be assigned to Coreductions as discussed, while the lower temperature (T_(max)=320° C.)peak corresponds to the reduction of oxide form of Pd (Pd²⁺) intoPd^(0.36) The peak area reduces with decreasing Pd content. Consistentwith previous reports, this suggests that Pd facilitated the Coreduction and improved the catalyst reducibility, which may beattributed to the hydrogen dissociation on surface Pd⁰ particlesfollowed by the successive spill-over of dissociated hydrogen atoms tothe Co species.³⁸

Both Pd-doped and un-doped perovskites showed higher reducibility thanthe CeO₂ and CeO₂—ZrO₂ (CZO) samples. For CeO₂, the main reductionhappened at much higher temperature starting at 500° C., with T_(max) ataround 800° C., which is assigned to Ce⁴⁺to Ce³⁺ reduction.³⁹ Theincorporation of Zr⁴⁺in CZO structure enhanced the reducibility of Cespecies, with the reduction peak shifting to lower temperature(T_(max)=520° C.). It is well established that in mixed ceria-zirconia asmaller ionic radius of zirconium favors the presence of Ce³⁺ ions byeliminating the strain associated with their formation, while theenhanced oxygen defects account for the improved reducibility/OSC.⁴⁰

3.4. Dynamic CO and Oxygen Storage Capacities for the Studied Materials

The CO and oxygen storage capacities of the studied materials weremeasured by dynamic isothermal redox cyclic tests, where CO and O₂ wererespectively used as the reducing and oxidizing agents. During themeasurement, constant flows of 20% CO/He and air were purged fortransient periods (30 seconds) in sequence through the degassed sample,and He was purged (for 15 min) in between the reduction and oxidationsteps to purge out the residual gas. The performance of each sample wasstudied isothermally at four redox temperatures (500, 600, 700, and 800°C.), while three repetitive measurements were performed in succession ateach temperature. The gas product composition downstream were closelymonitored by TCD and MS. FIG. 5A-5B and FIG. 6 illustrate the MSresponses (in terms of mass fraction partial pressure) of product CO₂(solid line peaks***) and unreacted O₂ (dotted line peaks***) during themeasurements for all the studied materials. Specifically, FIG. 5A-5Bcompares the material performances among different types of perovskites(PE-1 to 6) and the commercial OSMs (CeO₂ and CZO), while FIG. 6presents the OSCs of CaCo_(x)Zr_(1-x)O_(3-δ) perovskites (PE-6 to 9)with varying x values. Appreciable CO₂ formations (CO conversions) weredetected at the studied temperatures for all studied materials duringreduction. Generally, the redox properties increase with increasingtemperature from 500° C. to 800° C. In addition, the isothermal processshowed relatively steady responses for the CO conversion in thereduction steps and the O₂ storage in the subsequent oxidation steps,suggesting stable material performance.

In FIG. 6, the material redox properties improved with increasingtemperature from 500° C. to 800° C., and with increasing x values(higher Co content). At lower temperatures (500° C. and 600° C.),repeatable redox behaviours were shown with all studied materials,indicating stable OSC performance. As redox temperature was elevated(800° C.), CaCo_(x)Zr_(1-x)O_(3-δ)(when x>0.5) showed profound COconsumption during reduction, but continuously decreasing O₂ consumptionduring oxidation with cycles. This was likely due to the instability ofCo (CoO phase was released from the main phase through decomposition) athigh temperature reducing atmosphere.⁴¹ When higher amount of Zr waspresent in the perovskites (x<0.5), the oxygen storage was repeatableeven at higher temperatures. This is consistent with the previouslystated hypothesis that stabilized CaCo_(x)Zr_(1-x)O_(3-δ) perovskitestructures can be achieved by partial substitution of B-site Co withcertain amount of Zr (x≤0.5). When x=0.5, the optimum perovskitecomposition CaCo_(0.5)Zr_(0.5)O_(3-δ) exhibited optimum oxygen storagecapacity and stability.

3.5. Reaction Kinetic Study of Oxygen-Intake ofCaCo_(0.5)Zr_(0.5)O_(3-δ)

The oxygen intake kinetics of reduced CaCo_(0.5)Zr_(0.5)O_(3-δ)perovskite were further studied. FIG. 7A-7C shows plots used for kineticcalculation based on isothermal TG data between 350 and 475° C., andunder 5% O₂/He. Prior to measurement at each temperature, the sample wascompletely degassed at 500° C. and pre-reduced by 10% CO/He at the sametemperature. The data are analysed based on the solid-state kineticstudy methodology summarized by Hancock and Sharp,⁴² and recent relateddiscussions by Motohashi, et al.⁴³ The time-dependent fraction of thesolid reacted (α) and its variation (ln[−ln(1-α)]) are plotted as afunction of time on stream (t), reduced time t/t_(0.5), or time in logscale (ln t). The fraction reacted (α) is calculated using Eq. (4),where m₀, m_(t), and m_(final) are respectively the sample weight at thebeginning, time on stream of t (in min), and the end of TG measurementduring sample oxidation. A scale of 0-1 is allowed for the a value, withα=0 and α=1 respectively indicating the onset and the equilibrium statesof the solid reaction.

$\begin{matrix}{\alpha = \frac{m_{0} - m_{t}}{m_{0} - m_{final}}} & (4)\end{matrix}$

Generally, sample oxidation rate accelerates with increasing temperaturefrom 350° C. to 475° C. (FIG. 7A), and decreases with increasing time onstream by showing a smaller slope as t/t_(0.5) value increases for eachcurve (FIG. 7B). The reaction mechanisms can be examined by plottingln[−ln(1−α)] as a function of ln t (with t in min) when a linearrate-rated equation with slope m is acquired (FIG. 7C). The m values ofall the linear curves fall in the range of 0.6-1.3 (m=1.29, 1.23, 1.31,1.09, 0.88, 0.61 respectively at 350, 375, 400, 425, 450 and 475° C.).

The data points at 350° C.≤T≤475° C. nicely obey rate equation Eq. (5),indicating first-order kinetics for both samples at the studiedoxidation reaction conditions. Following Arrhenius equation, thereaction rate constants (k) in log scale upon oxygen intakes were thenplotted against inverse temperature (1000/T), as shown as FIG. 8. Thereaction activation energy (E_(a), in eV) values for the oxygen intakeprocess was calculated to be 0.159 eV for CaCo_(0.5)Zr_(0.5)O_(3-δ)perovskite.

−ln(1-α)=kt  (5)

3.6. Catalytic CO and HC Oxidation Activities of Pd-DopedCaCo_(x)Zr_(1-x)O_(3-δ)

Catalytic activities for CO and HC oxidation at simulated exhaust feedwith Pd-doped CaCo_(0.6-y)Zr_(0.4)Pd_(y)O_(3-δ)(y=0, 0.05 and 0.1)perovskite samples were investigated. C₃H₆ was used as the modelcompound for HC. FIG. 9 presents the catalytic oxidation activity offresh (a) CaCo_(0.5)Zr_(0.4)Pd_(0.1)O_(3-δ), (b)CaCo_(0.55)Zr_(0.4)Pd_(0.05)O_(3-δ) and (c) CaCo_(0.5)Zr_(0.5)O_(3-δ),with corresponding dopant Pd contents (wt-%) of 5.0%, 2.5% and 0%. Theoxidation activities were profiled as CO and C₃H₈ conversions vs.reaction temperature from 150-550° C., every 25 ^(o)C. The conversionprofiles were collected at three different stoichiometric numbers (SNs)at 0.95, 1.07 and 1.16, respectively simulating slight fuel rich,stoichiometric, and fuel lean conditions. As can be seen, all theperovskite-type OSMs exhibit excellent activities for CO and HC (C₃H₆)oxidation. Complete conversions of CO and HC were observed below 350° C.in all the Pd doped OSMs. The T₅₀ for C₃H₆ oxidation was as low as 250°C., while for CO oxidation was as low as 240 ^(o)C. The HC oxidationactivities of all the Pd doped CaCo_(x)Zr_(1-x)O_(3-δ)-TWCs werecomparable to, if not higher than, state-of-the-art TWCs. Thesepromising data clearly confirm the excellent potential for the proposedperovskite TWCs.

Generally, Pd-doped CaCo_(x)Zr_(1-x)O_(3-δ) samples showed highercatalytic oxidation activities than Pd-free one at all three conditions,especially at higher SN (fuel lean) conditions. It is obvious that thepresence of Pd promotes the surface chemisorption anddissociation/activation of CO and C₃H₈ molecules. The oxidation activitywere comparable with CaCo_(0.5)Zr_(0.4)Pd_(0.1)O_(3-δ), andCaCo_(0.55)Zr_(0.4)Pd_(0.05)O_(3-δ), suggesting mass transfer-limitationwith the later sample at the studied conditions. For the samePd-containing sample, higher conversions were shown at richer condition(SN=0.95), which were most likely because of more accessible activesites generated from “Pd segregation to the surface” at reducingconditions.^(27,29,36) The Pd-free CaCo_(x)Zr_(1-x)O_(3-δ) also showedCO and C₃H₈ conversions, and significantly enhanced conversions whenless O₂ was present in the feed (at lean condition). When Pd was absent,surface chemisorption became the rate-limiting step and CO and C₃H₈ fromthe atmosphere were likely directly oxidized by the surface oxygensnewly generated or transferred from the lattice. It is also interestingthat at fuel rich (SN=0.95), all samples showed reduced CO conversionsand increased H₂ productions at temperature above 400° C., suggestingthe occurrence of steam reforming reaction (excess C₃H₈ react with H₂Oproduct from oxidation reaction, to produce H₂ and CO) thermodynamicallypreferable (endothermic reaction) at higher temperatures. This could beavoided by operating engine mode at stoichiometric conditions.

4. CONCLUSIONS

In this disclosure, CaCo_(x)Zr_(1-x)O_(3-δ), (x=0, 0.3, 0.5, 0.7 and0.9) perovskites were synthesized for the first time and reported toshow improved redox property and oxygen storage capacity (OSC) comparedto the state-of-the-art perovskites and ceria-based oxygen storagematerials (OSMs). Pd-doped CaCo_(0.6-y)Zr_(0.4)Pd_(y)O₃ (y=0.05, 0.1 and0.2) samples showed promising catalytic activity towards C₃H₆ and COoxidation under simulated exhaust conditions, suggesting their potentialapplication in three-way catalysis for automotive emissions control.

The studied perovskites retain their main phase of orthorhombicLakargiite CaZrO₃ structure. Partial substitution of Co by Zr at B sitesenhances the perovskite structural crystallinity, but ZrO phase impurityincreased with increasing Zr content. For Pd-containing samples, Pd waspresent as both forms of bulk Pd²⁺ and surface Pd₀, and the amount ofsurface Pd⁰ increased with increasing Pd content. When x was around 0.5,perovskites exhibit optimum crystallinity and minimized impurity.

The redox and OSC properties were mainly attributed to B-site Co and Pd.The amount of reducible sites increased with increasing Co contents,while partial substitution with Zr enhanced the reduction of Co speciesby lowering the reduction temperature. Pd dopant also facilitated the Coreduction and improved the catalyst reducibility. Perovskite-type OSMwith composition of CaCo_(0.5)Zr_(0.5)O₃ with or without Pd doping givesthe optimized reducibility and structural stability. Further kineticsstudy showed a first order reaction mechanism with an activation energy(Ea) of 0.159 eV for CaCo_(0.5)Zr_(0.5)O_(3-δ).

Fresh CaCo_(0.5)Zr_(0.4)Pd_(0.1)O_(3-δ),CaCo_(0.55)Zr_(0.4)Pd_(0.05)O_(3-δ) and CaCo_(0.5)Zr_(0.5)O_(3-δ)samples all showed profound conversions of C₃H₆ and CO through catalyticoxidation at fuel lean-rich conditions (SN=1.16, 1.07 and 0.95), withlowest T₅₀s for C₃H₆ and CO conversions<250° C. Pd-doped perovskitesshowed higher oxidation activities than Pd-free one at all threeconditions, especially at higher SN (lean) conditions. For the samePd-containing sample, higher conversions were shown at richer condition(SN=0.95), which can be attributed to the reported phenomena of “Pdsegregation to the surface”.

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It should be understood that the description above is onlyrepresentative of illustrative embodiments and examples. For theconvenience of the reader, the above description has focused on alimited number of representative examples of all possible embodiments,examples that teach the principles of the disclosure. The descriptionhas not attempted to exhaustively enumerate all possible variations oreven combinations of those variations described. That alternateembodiments may not have been presented for a specific portion of thedisclosure, or that further undescribed alternate embodiments may beavailable for a portion, is not to be considered a disclaimer of thosealternate embodiments. One of ordinary skill will appreciate that manyof those undescribed embodiments, involve differences in technology andmaterials rather than differences in the application of the principlesof the disclosure. Accordingly, the disclosure is not intended to belimited to less than the scope set forth in the following claims andequivalents.

INCORPORATION BY REFERENCE

All references, articles, publications, patents, patent publications,and patent applications cited herein are incorporated by reference intheir entireties for all purposes. However, mention of any reference,article, publication, patent, patent publication, and patent applicationcited herein is not, and should not be taken as an acknowledgment or anyform of suggestion that they constitute valid prior art or form part ofthe common general knowledge in any country in the world. It is to beunderstood that, while the disclosure has been described in conjunctionwith the detailed description, thereof, the foregoing description isintended to illustrate and not limit the scope. Other aspects,advantages, and modifications are within the scope of the claims setforth below. All publications, patents, and patent applications cited inthis specification are herein incorporated by reference as if eachindividual publication or patent application were specifically andindividually indicated to be incorporated by reference.

What is claimed is:
 1. A catalyst comprising a platinum-group metal anda perovskite having the formula CaCo_(1-x)Zr_(x)O_(3-δ) wherein x is anumber defined by 0.02≤x≤0.98; and δ is a number defined by 0.0≤δ≤1.0.2. The catalyst of claim 1, wherein the platinum-group metal is Pd, Pt,Rh, Ru or a combination thereof.
 3. The catalyst of claim 1, wherein theplatinum-group metal is a combination of Pd and Rh.
 4. The catalyst ofclaim 1, wherein the catalyst is on an Al₂O₃ support, a titania support,a zirconia support, a ceria support, a silica support, an alumina-silicasupport, a zeolite support, or a carbon support.
 5. The catalyst ofclaim 1, wherein the catalyst is formed into a monolith honeycomb block.6. The catalyst of claim 1, wherein the catalyst is coated on to aceramic monolith honeycomb block.
 7. The catalyst of claim 6, whereinthe ceramic monolith honeycomb block is a cordierite compound.
 8. Thecatalyst of claim 1, wherein the catalyst is a three-way catalyst. 9.The catalyst of claim 1, wherein the catalyst is used to catalyze thereduction of NO_(x) or the oxidation of CO or hydrocarbons from aninternal combustion engine.
 10. The catalyst of claim 9, wherein theinternal combustion engine is an automobile engine.
 11. The catalyst ofclaim 9, wherein the internal combustion engine is operated understoichiometric air-to-fuel ratio conditions.
 12. The catalyst of claim9, wherein the internal combustion engine is fueled by diesel fuel,ethanol-gasoline hybrid fuel, gasoline or natural gas.
 13. The catalystof claim 12, wherein the ethanol-gasoline hybrid fuel is 85% ethanol 15%gasoline (E85).
 14. A method for reducing emissions from an internalcombustion engine which comprises contacting an exhaust stream from theinternal combustion engine with a catalyst comprising a platinum-groupmetal and a perovskite having the formula CaCo_(1-x)Zr_(x)O_(3-δ)wherein x is a number defined by 0.02≤x≤0.98; and 6 is a number definedby 0.0≤δ≤1.0.
 15. The method of claim 14, wherein the platinum-groupmetal is Pd, Pt, Rh, Ru or a mixture thereof.
 16. (canceled) 17.(canceled)
 18. (canceled)
 19. An exhaust system for reducing emissionsfrom an internal combustion engine which comprises the catalyst ofclaim
 1. 20. A perovskite catalyst having the formulaCaCo_(1-x)Zr_(x)O_(3-δ) wherein x is a number defined by 0.02≤x≤0.98;and 6 is a number defined by 0.0≤δ≤1.0.
 21. The catalyst of claim 20wherein x is a number defined by 0.2≤x≤0.8.
 22. (canceled) 23.(canceled)
 24. A method of preparing the perovskite catalyst of claim20, the method comprising: (a) dissolving salts of Ca, Co and Zr to forma homogenous solution; (b) drying the solution; and (c) calcining andsintering to form the perovskite catalyst.
 25. The method of claim 24,wherein the calcining is at about 300° C. to about 500° C. and thesintering is at about 800° C. to about 1400° C. 26.-59. (canceled)